/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.solr.search;

import java.io.IOException;
import java.util.Arrays;
import java.util.concurrent.atomic.AtomicBoolean;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.search.CollectionTerminatedException;
import org.apache.lucene.search.Collector;
import org.apache.lucene.search.FilterCollector;
import org.apache.lucene.search.FilterLeafCollector;
import org.apache.lucene.search.LeafCollector;
import org.apache.lucene.search.Sort;
import org.apache.lucene.search.SortField;
import org.apache.lucene.search.TopDocsCollector;
import org.apache.lucene.search.TopFieldCollector;
import org.apache.lucene.search.TotalHitCountCollector;

/**
 * A {@link Collector} that early terminates collection of documents on a per-segment basis, if the
 * segment was sorted according to the given {@link Sort}.
 *
 * <p><b>NOTE:</b> the {@code Collector} detects segments sorted according to a an {@link
 * IndexWriterConfig#setIndexSort}. Also, it collects up to a specified {@code numDocsToCollect}
 * from each segment, and therefore is mostly suitable for use in conjunction with collectors such
 * as {@link TopDocsCollector}, and not e.g. {@link TotalHitCountCollector}.
 *
 * <p><b>NOTE</b>: If you wrap a {@code TopDocsCollector} that sorts in the same order as the index
 * order, the returned {@link TopDocsCollector#topDocs() TopDocs} will be correct. However the total
 * of {@link TopDocsCollector#getTotalHits() hit count} will be vastly underestimated since not all
 * matching documents will have been collected.
 *
 * @deprecated Use {@link TopFieldCollector} and set trackTotalHits to false.
 * @lucene.experimental
 */
@Deprecated
final class EarlyTerminatingSortingCollector extends FilterCollector {

  /**
   * Returns whether collection can be early-terminated if it sorts with the provided {@link Sort}
   * and if segments are merged with the provided {@link Sort}.
   */
  public static boolean canEarlyTerminate(Sort searchSort, Sort mergePolicySort) {
    final SortField[] fields1 = searchSort.getSort();
    final SortField[] fields2 = mergePolicySort.getSort();
    // early termination is possible if fields1 is a prefix of fields2
    if (fields1.length > fields2.length) {
      return false;
    }
    return Arrays.asList(fields1).equals(Arrays.asList(fields2).subList(0, fields1.length));
  }

  /** Sort used to sort the search results */
  private final Sort sort;

  /** Number of documents to collect in each segment */
  private final int numDocsToCollect;

  private final AtomicBoolean terminatedEarly = new AtomicBoolean(false);

  /**
   * Create a new {@link EarlyTerminatingSortingCollector} instance.
   *
   * @param in the collector to wrap
   * @param sort the sort you are sorting the search results on
   * @param numDocsToCollect the number of documents to collect on each segment. When wrapping a
   *     {@link TopDocsCollector}, this number should be the number of hits.
   * @throws IllegalArgumentException if the sort order doesn't allow for early termination with the
   *     given merge policy.
   */
  public EarlyTerminatingSortingCollector(Collector in, Sort sort, int numDocsToCollect) {
    super(in);
    if (numDocsToCollect <= 0) {
      throw new IllegalArgumentException(
          "numDocsToCollect must always be > 0, got " + numDocsToCollect);
    }
    this.sort = sort;
    this.numDocsToCollect = numDocsToCollect;
  }

  @Override
  public LeafCollector getLeafCollector(LeafReaderContext context) throws IOException {
    Sort segmentSort = context.reader().getMetaData().sort();
    if (segmentSort != null && canEarlyTerminate(sort, segmentSort) == false) {
      throw new IllegalStateException(
          "Cannot early terminate with sort order "
              + sort
              + " if segments are sorted with "
              + segmentSort);
    }

    if (segmentSort != null) {
      // segment is sorted, can early-terminate
      return new FilterLeafCollector(super.getLeafCollector(context)) {
        private int numCollected;

        @Override
        public void collect(int doc) throws IOException {
          super.collect(doc);
          if (++numCollected >= numDocsToCollect) {
            terminatedEarly.set(true);
            throw new CollectionTerminatedException();
          }
        }
      };
    } else {
      return super.getLeafCollector(context);
    }
  }

  public boolean terminatedEarly() {
    return terminatedEarly.get();
  }
}
